. on, A. C. Graves June 28, 1951 7. Our lack of knowledge of the effects of retention of a given number of active particles in the lungs devs not permit a dividing line to be sect up between a harmful and a harmless concentration. We are sure only that a zero concentration is harmless, and a zero concentration cannot be guaranteed. In view of paragraph 6 above, one mayexpect appreciable concentrations of small particles at any place and at my time beyond some minimum following a shot. 8, Our knowledge of the effects of external y- and @G -dosage is considerably more precise than is that of the effects of inhalation or ingestion. Damage from both y- and /-radiation may be expected from exposure to the products of a nuclear explosion, The radiations will be quantitatively related and the more serious will be the y-radiation. The severe -burns noted on cattle near Trinity are a strong indication that these same cattle were subject to @-doses of the samo order as the emergency tolerance and possibly higher. For the present, the gy~dose is the best criterion for judging the degree of radio- logical hazatd. Im the case of sporadic exposure of the kind contemplated, in contrast with the repeated regular exposure suffered by workers in radiological fields, the allowable dose can, from the safety point of view, very well be raised to 5 or 10 roentgens (publicity considerations disregarded), GC, The theoretical model used to predict radiation levels as a function of distance for various conditions of particle size, cloud height, and wind veLocity is based on the following assumptions: 1. The wind is constant in velocity and direction from the surface to the top of the cloud. 2. Directional and velocity wind shears are implied in the assumption that the cloud spreads horizontally 1 mile in 6. 3. Stoke's Law governs the rate of fall of all particles of interest. h. ‘The activity in the cloud at the time it begins to move away from the site is more concentrated at the top than at the surface. At any height the activity is initially proportional to h//?, 5. Thefraction of the total activity carried by particles of dia- moter between D and D + dD is given by dA where 0 2 Kx%a7* qx x=2 a and "a" is a parameter representing a mean particle size, and "K" is a normalizing constant. eee